A diffractive optical element (“DOE”) is a physical element that redirects selected wavelengths of light into specific positions that are defined by the physical properties of the element. Typically, these DOEs are used to split a single beam of light into multiple beams of light. Such conventional DOE splitters are often used in such applications as bar code scanners, compact discs, or a variety of laser scanning devices.
A DOE can be reflective or transmissive. A reflective DOE splitter receives one incoming light beam and reflects it onto multiple light beams. In contrast, a transmissive DOE receives one incoming light beam and allows it pass through the DOE to diffract it onto multiple light beams.
A DOE splitter will actually split the incoming beam into a large number of diffracted orders, with the higher reflected orders generally dropping off in power. But any given DOE splitter application will not use all of these reflected orders. Rather, a DOE splitter will be designed with a specific number of primary output beams. These primary output beams (i.e., primary diffracted orders) have their power levels controlled by the DOE design to be very similar, while the remaining output beams (i.e., the secondary diffracted orders) will have power levels kept as low as possible. However, because only a subset of the total diffracted orders are used as output beams (i.e., only the primary orders), DOEs have an inherent limit on their efficiency. Any power diffracted into the unused beams (i.e., the secondary diffracted orders) is lost, dropping the DOE's efficiency below 100%.
DOE splitter applications have fairly demanding requirements with respect to the uniformity of the power for the primary diffracted orders (i.e., the maximum allowable variation of the split beams). In other words, the DOE splitters are required to output a number of beams having power levels that are very similar to each other. Generally these applications allow less than a few percentage of peak-to-peak variation in the power of the diffracted beams.
One type of conventional DOE splitter is based on a simple binary design. In other words, such DOE splitters are based on patterns of steep walled grooves with a single etch depth. These binary DOE splitters typically have a relatively modest efficiency (e.g., in the range of 80%). Alternate DOE splitter designs have been suggested that use multi-level etch depths. But these also include steep walls for the etching patterns. And they can typically only raise efficiency up to about 90%.
Furthermore, the surface patterns used in these conventional DOE splitters are discontinuous etch profiles, which limit the ability to deposit additional layers on the DOE subsequent to surface etching. As a result, when reflective DOEs are used, the discontinuously etched shapes may degrade in performance in reflection because the thick high reflection coating (typically 5 μm or more thick) will not conform to the etched surface shape near discontinuities or regions of large slope. Similar issues arise for transmissive DOEs, on which non-reflective coatings may be applied.
It would therefore be desirable to provide a DOE that can achieve a higher efficiency and will allow for the deposition of additional layers after etching without degrading performance. Furthermore, it would also be desirable to provide a DOE that is suitable for combiner applications.